The easiest way to see performance increase without Formatting your PC.
Download CCleaner as mentioned before. http://www.piriform.com/ccleaner
Clear all your history with it. This will delete all the crap that has accumulated on your PC over time (Be careful though because you'll lose saved passwords and auto complete if you check the wrong boxes.
With the same Software you can uninstall any unwanted Programs and Bloatware to free up HDD space.
There's also an option within CCleaner to clear Start Up programs. Look through the list and delete any entries that aren't needed. you can basically delete pretty much everything in there, as Windows will override and start up any essential stuff. Don't delete iTunes Helper though. Cleaning up stuff that starts with Windows helps your PC boot into the windows desktop faster as it uses less resources.
If you don't already use it, download Microsoft Security Essentials. It uses significantly less system resources than most other Antivirus software. Make sure you completely Uninstall any previous software first. Norton can be pretty tricky to completely Uninstall.
http://www.microsoft.com/en-gb/security ... y/mse.aspx
Another thing that could help is checking to see if you have any shitty toolbars slowing down your browser.
After you've done all that, Download Defraggler from the same Website as CCleaner. Install that and Defrag your Hard Disk.
If it still runs slow you're going to need to do a fresh install of Windows. It's easier than you might think. Post what size hard drive you have and how much space you've used on it and we'll give you an idea of how many DVD's you'll need or what size external Hard Drive. You may even be able to use a larger capacity Flash Drive.
Do you have a second Computer/Laptop?
“Life is like riding a bicycle. To keep your balance, you must keep moving.” - Albert Einstein
“Life is what happens to you while you're busy making other plans” - John Lennon